In the method for culturing microorganisms such as yeast, bacteria, etc., under aerobic conditions to produce microbial cells and for producing useful substances such as antibiotics, amino acids, alcohols, etc., research and development was made about the measures for providing oxygen and for cooling culture broth heated during a fermentation process, etc., in both batch and continuous fermentation systems for the purpose of improving fermentation efficiency. For instance, a method for controlling the temperature of a culture broth by providing a cooling tube inside the fermenter is known. However, the diffusion efficiency of air provided in the fermenter into the culture broth is lowered in this method and, in addition, it has also the defect that the effective area inside the fermenter is reduced. Therefore, a fermentation process using a stirred gas bubble fermenter provided with a cooler and a device for eliminating bubbles outside the fermenter has also been proposed (ex. Japanese Patent Publication Gazette No. 26040/1964).
By way of illustration, in controlling a culture broth to an appropriate temperature by cooling it, it is necessary to remove bubbles in the culture broth passed into a cooler as much as possible in order to obtain a good cooling effect.